1. Field of the Invention
The raw material for animal improvement is genetic variation. Increasing genetic variability as a base for increased progress in animal breeding has been a long-term goal of agricultural research. Recent advances in molecular genetics open the possibility for transferring new genetic information to the germ line of various species. Desirable genes could then be introduced into an improved line of domestic animals and these strains used for further improvement. For example, in chickens, egg production strains are highly susceptible to avian leukosis virus (ALV). ALV is a common retrovirus of chickens that induces a B cell lymphoma called lymphoid leukosis and other neoplasms and also results in losses of productivity in mature chickens. The initial steps in the infection of chicken cells by ALV is the attachment of the retrovirus envelope glycoprotein to the cell membrane and transport of virion contents to the cytoplasm. The specificity of binding or penetration of the virion is determined by the viral envelope glycoprotein. This specificity has been used to classify ALV into different subgroups by a phenomenon called interference. Retroviruses are prevented from infecting chicken cells that were previously infected with the same subgroup by specific physical inhibition of viral absorption or penetration.
Efforts to reduce the rate of congenital infection are underway by commercial breeders. Single recessive genes for resistance to infection by each subgroup of ALV exist in chickens, but their frequency is low in egg-producing strains, and few resistant commercial strains have been developed. Introduction of a dominant gene for resistance to ALV infection in available egg production strains would achieve this goal and at the same time demonstrate that other economically important new genes can be introduced into the genome of a food animal species without disrupting the breeding program.
2. Description of the Prior Art
Natural insertion of genetic information into the chicken germ line has been occurring since the speciation of the chicken [Frisby et al., Cell 17: 623-634 (1979); Astrin et al., Cold Spring Harbor Symposium 44: 1105-1109 (1980); and Hughes et al., Cold Spring Harbor Symposium 44: 1077-1091 (1980)]. At least 22 endogenous viral genomes (proviruses) have been identified and characterized in the White Leghorn. Many more exist in other commercial lines of chickens [Hughes et al., Virology 108: 222-229 (1981); Gudkov et al., J. Gen. Virol. 57: 85-94 (1981)]. The presence of these proviruses suggests that there is a natural mechanism for proviral integration into the germ line. Presumably, retroviral infection of germ cells occurs on rare occasions.
Crittenden et al. [Avian Dis. 30(1): 43-46 (1985)] proposed a method for providing resistance to subgroup A ALV based on an endogenous ALV model of interference. Chicken embryo fibroblasts (CEF) expressing subgroup E envelope glycoprotein coded for by the defective endogenous proviral genes, ev3 and ev6, are many times more resistant to subgroup E Rous sarcoma virus (RSV) infection than CEF lacking these genes. Furthermore, chickens carrying ev3 and ev6 are resistant to subgroup E ALV infection [Robinson et al., J. Virol. 40: 745-751 (1981)]. Salter et al. [Poult. Sci. 65: 1445-1458 (1986)] have previously described the insertion of avian leukosis proviral DNA into the germ line of the chicken using recombinant and wild-type ALV belonging to subgroup A. By injecting infectious retrovirus into fertile eggs at the day of incubation and testing the resulting viremic males for genetic transmission or proviral DNA to their progeny, they conclusively showed that proviral DNA had been inserted into the chicken germ line.